kc3-lang/libtommath/mp_prime_fermat.c

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• Hash : 448f35e2
  Author :
  Date : 2019-10-29T20:07:29
  

  simplifications: prime functions

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• mp_prime_fermat.c

• #include "tommath_private.h"
  #ifdef MP_PRIME_FERMAT_C
  /* LibTomMath, multiple-precision integer library -- Tom St Denis */
  /* SPDX-License-Identifier: Unlicense */
  
  /* performs one Fermat test.
   *
   * If "a" were prime then b**a == b (mod a) since the order of
   * the multiplicative sub-group would be phi(a) = a-1.  That means
   * it would be the same as b**(a mod (a-1)) == b**1 == b (mod a).
   *
   * Sets result to 1 if the congruence holds, or zero otherwise.
   */
  mp_err mp_prime_fermat(const mp_int *a, const mp_int *b, bool *result)
  {
     mp_int  t;
     mp_err  err;
  
     /* ensure b > 1 */
     if (mp_cmp_d(b, 1uL) != MP_GT) {
        return MP_VAL;
     }
  
     /* init t */
     if ((err = mp_init(&t)) != MP_OKAY) {
        return err;
     }
  
     /* compute t = b**a mod a */
     if ((err = mp_exptmod(b, a, a, &t)) != MP_OKAY) {
        goto LBL_ERR;
     }
  
     /* is it equal to b? */
     *result = mp_cmp(&t, b) == MP_EQ;
  
  LBL_ERR:
     mp_clear(&t);
     return err;
  }
  #endif
  

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